1. Field of Invention
This invention relates to laser communication systems and methods of operation. More particularly, the invention relates to laser communication systems and methods of operation using multiple transmitters and multiple receivers with dispersive multiplexing in multimode fiber.
2. Background Discussion
In laser communication systems, the information capacity of multimode fiber, from a fundamental standpoint, is greater than that of single mode fiber. In practice, however, modal dispersion makes it impossible to use that capacity in any straightforward manner. However, the capacity of laser communication systems using multimode fiber can be increased by borrowing from recently developed techniques in wireless communications. In wireless systems, radio waves do not propagate simply from transmit antenna to receive antenna, but bounce and scatter randomly off objects in the environment. This scattering is known as multipath and it results in multiple copies of the transmitted signal arriving at the receiver via different scatter paths.
A technique known as BLAST (Bell Labs Layered Space-Time) utilizes multipath effects in a scattering environment to increase the capacity of a wireless channel. BLAST exploits multipath by using the scattering characteristics of the propagation environment to enhance, rather than degrade transmission accuracy by treating the multiplicity of scattering paths as separate parallel subchannels. BLAST enhances multipath by splitting a single user""s data stream into multiple substreams and using an array of transmitter antennas to simultaneously launch parallel substreams. All the substreams are transmitted in the same frequency band, so spectrum is used very efficiently. Since the user""s data is being sent in parallel over multiple antennas, the effective transmission rate is increased roughly in proportion to the number of transmitter antennas. At the receiver an array of antennas is again used to pick up the multiple transmitted substreams and their scattered images. Each receiver receives all of the transmitted substreams superimposed, not separately. However, if the multipath scattering is sufficient, then the multiple substreams are all scattered slightly differently since they originate from different transmit antennas that are located at different points in space. Using sophisticated signal processing, these slight differences in scattering allow the substreams to be identified and recovered. Further details on BLAST are described in an article entitled xe2x80x9cLayer space-time Architecture for Wireless Communication in a Fading Environment When Using Multiple Antennasxe2x80x9d by G. J Foschini, Bell Labs Technical Journal, Vol. 1, No. 2 Autumn 1996, pages 41-59.
Since multimode fibers mirror multipath effects in a wireless communication system, the capacity of the multimode fiber can be increased by modulating N data streams onto the same RF carrier with each data stream modulating a laser coupled to a multimode fiber. At the receiving end, the signal is split into M fibers that feed M receivers where M is greater than or equal to N. The signals received at each receiver will contain power from all the transmitters. If the individual received signals are uncorrelated, the same signal processing algorithms used in the BLAST architecture can be used to recover the N data streams.
An object of the invention is a laser communication system and method of operation combining transmitter/receiver diversity and modal dispersion to increase capacity without having to increase transmission speed or WDM channels.
Another object is a laser communication system and method of operation using multiple lasers and multiple detectors in conjunction with multimode fiber to improve transmission characteristics of the fiber.
Another object is transmitting N parallel data streams over a single multimode fiber in a laser communication system and distributing uncorrelated signals to M receivers where M is greater than or equal to N and each receiver receives all N signals.
These and other objects, features and advantages are achieved in a laser communication system and method of operation combining transmitter/receiver diversity and multimode fiber dispersion. Multiple transmitters and multiple detectors are coupled to the multimode fiber. The transmitters and detectors are coupled to the multimode fiber through xe2x80x9carmsxe2x80x9d which can consist, for example, of N optical fibers joined to the main fiber through a 1xc3x97N fiber splitter, where N is the number of data signals. Alternatively, the xe2x80x9carmsxe2x80x9d can be formed by either butt-coupling or free-space optical coupling to the fiber of the multiple lasers and detectors either in individual device or array configuration. The system makes use of the inherent modal-coupling diversity between the xe2x80x9carmsxe2x80x9d introduced by the optical system. The diversity, when combined with modal dispersion in the multimode fiber, introduces uncorrelation in both the transmitted and received signal arrays. A set of N data streams is RF modulated onto RF carriers of identical frequency. Any RF modulation format can be chosen though the identical format must be used for all N data streams. The N RF modulated data streams are used to intensity modulate the N laser transmitters. The N optical signals are combined onto the single multimode fiber for transmission of an optical signal to the multiple detectors. The optical signal at the end of the multimode fiber is fed, by one of the means described above, to M detectors, where M is greater than or equal to N. Each M detector receives power from all N transmitting lasers. The M detectors utilize direct optical detection to convert the received optical signal to a RF signal, which can be demodulated using standard RF techniques. A known set of bits is sent at the start of each transmission to allow the receiver to estimate the Mxc3x97N transmission matrix. The uncorrelation introduced by the transmitter and receiver diversity allows the original N data streams to be recovered by signal processing algorithms. Since the data streams are being sent over multipaths, the effective transmission rate is increased, roughly in proportion to the number of transmitters. Because the modulated RF carrier frequencies are identical for each transmitter, this increased transmission rate requires only the electrical bandwidth used to modulate one of the transmitting lasers.